1. Field of the Invention
This invention relates to a filter circuit and, more particularly, to a so-called double integration type filter circuit consisting essentially of a pair of integrators connected in series with each other.
2. Description of the Prior Art
In general, in a process of checking an electronic circuit, it may become necessary to adjust the peak frequency, dip frequency or the cut-off frequency of the filter circuit to a prescribed target value. Above all, in a circuit formed in an analog IC, the circuit elements such as transistors, resistors or capacitors may be fabricated with a rather accurate relative ratio of the rated values thereof, but the magnitudes of these rated values usually vary from circuit to circuit. Hence, the aforementioned adjustment is thought to be indispensable in view of a demand for higher accuracy in the filter circuit.
It is noted herein that a double integration type filter circuit as disclosed, for example, in the U.S. Pat. No. 4,472,689, can be built into an IC. The double integration type filter circuit has advantages that a higher accuracy suitable for incorporation into an IC may be realized and that it allows various filter characteristics, including the value of Q.sub.1 to be adjusted more easily.
It is noted that, at the time of adjusting the filter circuit, it has so far been necessary to detect portions proper to the characteristic curves, such as so-called cut-off points or peak portions, and to adjust the frequencies of these portions, namely the so-called cut-off or peak frequencies, to prescribed target values. However, it is troublesome or even difficult with the LPF, HPF or the BFP with is low value of Q to locate the cut-off or peak points accurately on the characteristic curves.
Above all, when it is desired to automate such filter adjustment, it is difficult to discriminate and read out mechanically those characteristic portions, such as the aforementioned cut-off points, from the frequency characteristic curves, thus occasionally resulting in lowered adjustment accuracy.